Kentucky Grape Diseases

Chris Smigell, UK Extension Associate

The most common situations where disease develops are:

1. where growers have let 14 days or more lapse between sprays
2. where canopies are very dense, usually because shoots have not been thinned and/or combed down.
3. powdery mildew commonly develop in Vidal blanc, when the clusters and shoots have not been thinned.

In dry years, disease problems occur when growers allow 14 or more days between fungicide sprays, particularly in May and June.

In wet years, keep a close spray schedule.  Hopefully there are enough day in between rains to allow the ground to dry, allowing growers to go into the fields (especially on the slopes) and get the sprays on and the have a couple hours for the sprays to dry.

Once you get some SUCCESSFUL experience with fungicides, you can experiment
with going longer between sprays, but only in mid- to late summer, AND ONLY when weather permits.

Here some pictures, to help you identify what you may be seeing in your vineyard.

Picture 1. Anthracnose lesions

A picture of anthracnose lesions (1.), taken from the OSU Midwest Grape Production Guide. Lesions have cracked, bleached or tan centers, and are mainly round to rectangular, and have dark, raised borders. Phomopsis cane and leaf spot also has lesions with dark borders, and tan centers, but phomopsis lesions usually are elongated,
and can have superficial, tan scabbing. Phomopsis lesions tend to be seen in the first 1-4 internodes from the base of the shoots, but this is not always true.

Picture 2.  anthracnose or phomopsis lesions

Picture 2 shows lesions that could be from either anthracnose or phomopsis.

Picture 3.  anthracnose or phomopsis lesions

Picture 3 shows a leaf commonly seen near the base of a shoot in the early season. Again, either disease could cause this. Typically, anthracnose lesions lack the yellow halos that phomopsis lesions have, but both lesions can be numerous, tiny, (1-5 mm across), and may or not be grouped along leaf veins. Lesions of both also can dry out and be open , i.e., a shot-hole appearance. Dying leaf tissue can result from either disease. You may begin seeing these symptoms in mid May.

Picture 4.  Typical phomopsis

Picture 4 shows what is typical of phomopsis, with the halos and scabbed over black lesions. It may require lab diagnosis to figure out which fungus you have, if not both. It is
important to know which you have, as the control measures are a bit different. By fruit set, you may see the clincher symptom of anthracnose, birds eye rot, (Picture 5). Phomopsis will also infect berries, but the fungus does not cause disease until late in the
season, near ripening.

Picture 5.  Berries with anthracnose, bird's eye rot.

Picture 6. Black Rot

Picture 6 is a progression of the black rot disease in grape berries. This can happen in a few days, and that is why growers usually get taken by surprise by this disease.

Picture 7.  Close up of mummified berry.

Picture 7 is a close up showing the diagnostic, tiny, black pycnidia in a mummified berry. Phomopsis-diseased berries also have pycnidia, and are shriveled, but you would not expect to see this at this time of year. Watch for this during ripening.

Picture 8.  leaf lesion from pycnidia

Pycnidia also appear in the centers of leaf lesions (Picture 8). Gramoxone and other chemical burns may have a chestnut color and similar size, but the pycnidia in the centers are diagnostic.

The OSU Midwest Grape Production Guide has good pictures Downy Mildew and Powdery MIldew.

What do I do if I already have these diseases?

Anthracnose and Phomopsis are primarily spring diseases, and the critical time for control is past. Berry infections by them will be controlled to varying extents by the fungicides you use to control Powdery and Downy mildews and black rot.

Even if the crop is a total loss, or the crop in a section of a planting is completely ruined, these vines will need to be sprayed with fungicides, to prevent worse damage to the leaves, and to minimize the amount of fungus spores that can drift to healthy blocks of vines. Even though you continue to spray those healthy vines, the lesser the load of spores they receive, the lesser the risk of any infections (and resultant disease) occurring.

First, open up the canopy if it is dense. This allows faster drying of night dew and rain (both favoring fungal growth), and allows better spray penetration. Light penetration is also important, as it tends to kill spores. (It will also improve fruit quality.) Although it is about a month late to do so, it would help to thin out shoots and clusters. Thin to 4-6 shoots per foot of cordon. Thin out fruit clusters that are bunched together. At the same time, you can remove diseased clusters, if the planting is small enough.

Also make sure that vertically trained shoots in the VSP or Kniffen or Smart Dyson trellises are positioned well in the catch wires. Heavy winds commonly push the shoots into bunches, prohibiting spray and sunlight penetration. For GDC and high cordon trellises, separate and comb down shoots.

HOWEVER, while it may make sense to cut these shoots short to allow air circulation, only trim back enough to keep the shoots off of the ground.

Leave as many leaves beyond the clusters as possible, to maximize fruit sugars.

Hedging the vines will remove lateral shoots growing from the main shoots. Leaf pulling near the clusters will also help, but this needs to be done well before veraison, to avoid scalding of fruit. For details on these procedures, see the OSU Midwest Grape Production Guide, in its full color entirety at http://ohioline.osu.edu/b919/index.html. Better yet, buy a copy, and keep it handy. It's cheap.

The next thing to do is choose the right fungicides to halt further infection.

Below is a list borrowed from Dr. John Hartman. It includes some combination sprays that can help halt disease progression. REMEMBER: there is a 66 day pre-harvest interval for mancozeb formulations, like manzate and dithane. You should stop using these now if you anticipate harvesting in the first couple weeks in September (table grapes, Foch, some others).

• Use of Quintec plus mancozeb at this time will help with powdery mildew (Quintec) and black rot, downy mildew, Phomopsis, and Botrytis (mancozeb).
• Use of Pristine now would provide powdery mildew, downy mildew, and black rot control plus protection against Botrytis.
• Rally plus captan will provide benefits against powdery mildew, and black rot (Rally) and downy mildew and Phomopsis (captan).
• Abound plus sulfur prevents powdery mildew (sulfur), black rot, and downy mildew (Abound).
• Rubigan plus mancozeb prevent powdery mildew and black rot (Rubigan) plus downy mildew and Phomopsis (mancozeb).
• Be aware that products containing mancozeb will be expensive and in short supply for this growing season.

updated:  07/15/2009